This research proposal is concerned with: a) how the roles of recruitment and/or rate coding vary in the modulation of voluntary contractions in human muscles of different muscle fiber type compositions, b) evaluation of EMG/force relations in these muscles, c) detection and quantification of synchronous motor unit firing and its effect upon the surface EMG, and d) determination of the factors influencing shivering and physiological tremor, with the aim of determining which mechanisms are common to these two conditions and those which differ. The objective is to analyze the motor outflow so that the more general neural control can be investigated. Since both conditions are likely sources of easily detectable neuronal synchronization, they may serve as models of the synchronized state to which other situations of less apparent neuronal synchrony may be compared. Intramuscular records of single motor unit activity, together with surface EMG and force measurements will be obtained. Statistical techniques have been worked out for evaluating the correlation between motor unit pairs and the quantification of synchronization present in the surface EMG. Individual spike trains will be further analyzed to investigate the discharge patterns of single motor units over large force range of a voluntary muscle contraction. Motor unit classification will be made according to their contractile properties obtained from spike-triggered averaging as well as the recruitment threshold and location of the units within the muscle. These will be compared with histochemical studies.